Medical devices, such as cardiac pacemakers and implantable cardioverter defibrillators (ICDs), provide therapeutic electrical stimulation to a heart of a patient via electrodes carried by one or more medical electrical leads and/or electrodes on a housing of the medical device. The electrical stimulation may include cardiac pacing pulses and/or cardioversion/defibrillation (CV/DF) shocks.
The medical device may sense cardiac electrical events attendant to the intrinsic heart activity for detecting an abnormal intrinsic heart rhythm. Upon detection of an abnormal rhythm, such as bradycardia, tachycardia or fibrillation, an appropriate electrical stimulation therapy may be delivered to restore or maintain a more normal rhythm of the heart. For example, an ICD may deliver pacing pulses to the heart of the patient upon detecting bradycardia or tachycardia or deliver CV/DF shocks to the heart upon detecting tachycardia or fibrillation.
The ICD may sense the cardiac electrical signals from a heart chamber and deliver electrical stimulation therapies to the heart chamber using endocardial electrodes carried by transvenous medical electrical leads. In other cases, a non-transvenous lead may be coupled to the ICD, in which case the ICD may sense cardiac electrical signals and deliver electrical stimulation therapy to the heart using extra-cardiovascular electrodes. Cardiac event signals, such as atrial P-waves and ventricular R-waves, sensed by electrodes positioned within the atrial or ventricular heart chamber from which the signal arises, generally have a high signal strength for reliably sensing the cardiac electrical events. In other examples, a non-transvenous lead may be coupled to the ICD, carrying electrodes positioned at extra-cardiovascular locations, in which case the cardiac event amplitudes may have a relatively lower or more variable signal strength and/or different relative amplitudes between R-waves, T-waves and P-waves in the cardiac electrical signal. Reliable sensing of cardiac events by a pacemaker of ICD is important in determining when an electrical stimulation therapy is needed and delivering the appropriate electrical stimulation therapy for treating an abnormal heart rhythm.